1. Field of the Invention
The present invention relates to a humidity sensor employed for measuring humidity in the atmosphere or humidity control.
2. Description of the Related Art
Conventionally, for the measurement of humidity, there have beem employed a hair hygrometer, wet and dry bulb hydrometers or the like. However, due to the recent progress in the field of electronic techniques, electrical measurements are brought into application due to simplicity in operation. In the electrical measurements referred to above, materials whose resistance values or dielectric constants vary depending on humidity are employed to convert the humidity variations into quantities of electricity due to variations in the resistance values or dielectric constants. By way of example, as the materials whose resistance values are altered, metallic oxides or ceramics are employed, while for the materials whose dielectric constants are varied, organic films may be used. However, since these humidity sensors are arranged to detect the humidity through contact thereof with the atmosphere, there has been a disadvantage in that the characteristics of the humidity sensors are deteriorated by adhesion of atmospheric pollutants to the surfaces of the humidity sensors.
Therefore, it has been a recent practice to prevent the deterioration of the humidity sensor characteristics by a heating regeneration method in which harmful substances adhering to the surface of the humidity sensor are removed by heating the sensor at high temperatures or by a method in which the humidity sensor is covered by a surface soiling prevention filter. Such a surface soiling prevention filter is mainly applied to an organic film type sensor which can not be subjected to the heating regeneration, while the heating regeneration method is utilized for the humidity sensor employing ceramics.
The humidity sensor to which such heating regeneration is applied will be described hereinbelow with reference to FIG. 10.
In FIG. 10, an electrode 3 is fixed on a surface of a humidity sensing portion 1 composed of ceramics and the like, while a heater 2 is attached to its surface confronting this electrode 3. The humidity sensing portion 1, heater 2 and electrode 3 are fixed to a holding portion 4, and a protector 5 enclosing these parts for protection thereof against mechanical damages is secured to the holding portion 4. This protector 5 is formed with a large number of through-holes 5a in its peripheral face for improved heat radiation. Moreover, lead wires 6 electrically connectioned with the electrode 3 are connected with the electrode 3 through the holding portion 4.
When the humidity sensor having the construction as described above is brought into contact with the measuring atmosphere, water vapor in the atmosphere arrives at the humidity sensing portion 1 by diffusion or by air stream so as to be absorbed thereonto, and depending on the amount of absorption at that time, the humidity sensing portion 1 undergoes a variation in its electrical characteristics. If the variation is preliminarily applied in the form of voltage to the electrode 3 through the lead wires 6, humidity in the atmosphere may be electrically detected. Meanwhile, when the characteristics of the humidity sensor are deteriorated by the adhesion of harmful substances onto the surface of the humidity sensing portion 1, it has been a practice to restore the characteristics by heating the humidity sensing portion 1 through energization of the heater 2 so as to burn the adhering harmful substances into the form of a gas for removal.
However, in the conventional humidity sensor, harmful substances are also drawn in together with the atmosphere via the through-holes 5a of the protector 5 to adhere to the humidity sensing portion 1. Such harmful substances may be broadly divided into the harmful substances of organic nature such as soot, smoke of cigarettes, etc. and also into those of inorganic nature such as salt particles, dust, etc. Although removal of organic harmful substances can be effectively made by the heater 2, such heating is hardly effective for the inorganic harmful substances such as salt particles, dust and the like which can not be formed into a gas by combustion for removal. Moreover, since the heater 2 is subjected to high temperatures above 500.degree. C., the protector covering the heater 2 is also heated up to a high temperature, and if any combustible material is located close to the protector 5, there is such a danger that the combustible material is ignited. Furthermore, there is also such a problem in that, if the humidity sensor is provided in a combustible gas atmosphere, an explosion may take place by the heat of the heater 2.